Urea synthesis



Patented Mar. 24, 1953 UREA SYNTHESIS Delaware ,Y

L:'Ihis;1invention relates tothe synthesis of urea. Inria Kspecicfaspeet this invention relates to a novel` method forewithdrawing urea from a regactorforthe synthesis of urea. In another spec'icz 4aspect this invention relates to` a. novel method for effecting the synthesis of urea wherein the urea is formed and crystallized in the same reactor. i Y "Urea is producedby the interactionV of ammonia andz carbon `.dioxide or compounds thereof, Vwith or :without water and with or without dehydrating agents, athigh temperatures and pres- .suresiwithink a closed vessel. A melt is produced which is highly corrosive in nature and which is characterized by arelatively high autogenously developed"pressure. In the past; diicultyhas been encountered in removing the melt from the reactor'. Valves have been employed to exercise control over the highv pressure, but the high temperature and pressure of the melt causes rapid corrosion and erosion of the valves. The cost of replacement parts and of operational shut-downs attendant therewith are undesirable economically, "and`V it is ani object of this invention to provide a method wherein the corrosion problem is at least partially eliminated.

Further, thelltemperature employed for `the synthesis of urea is sufficient to maintain" the urea in a liquid state, and to separatethe'urea of thekfreactor' atatemperature below that at 1urea crystallizes. ,IYv haveA also found b, withdrawing,th-urea ,QryStalSl imm, ih? y `A ta temperature suitable for'the'formaif Ytio of Vreacrystals througha ,valve across which therelisonly a "relatively small or vno[pressure vd i erential, the A corrosion` ,and erosion of. the

:s114128 markedlyredi- The figure is a diagrammatq representation of one g; irietlfiod ,of practicing my invention. 7 7Such gcriientipnaigseuipment as, ,pumps liompifssrs and, if itis, desiredto remove .the melt fro Donald I.,White, Bartlesville,` Okla., Vassigner to z .Phillips Petroleum Company, a corporationof hppliestionNovemben, 1949, serisiNanasr i h ,eel-aims( (C1. 26o-555)' 2 and the like` havenot been included in the -acicompanying drawing, but the ,inclusion of 'such equipment isbelieved to be well within the scope of my invention.

In practicing my invention, urea is formed from ammonia and carbon dioxide in a high pressure reactor orautoclaveat conditions similar to those practicedin the art. 'For eiiamplaxthe reaction may be 'eiected ata tervnperature of 300 to 400F., preferablyl 340 to 400"l F.,at vthe autogenous pressure ofthe synthesis, This pressure may vary from A170,0 to 3500 pounds per square inch absolute,A but pressures above the autogenous pressure and as high as 8,000 pounds per square inch absolute may be employed. The reactor is preferably a vertically,` elongated autoclave, and it is so constructed that the upperportion of the reactor contents is Vrr'iainta'inecl atja temperature within Ythe above#named` ranges. The lower portion ofv thel'reactor contents,` Y is maintained at au temperature below the 'melting point ofv urea y(132.7?0. 278.6" F.), preferably^at ,al temperature below 140 F., and 'more prefer'- ably at a temperature .within the range ofl'OO to "l F, Attheseoperating conditions al thick slurry'containingurea formsjin the lowerportion of the reactor,` ,Thefthck slurry Vis withdrawn` f romthe lowerL portion of thereactor, and, since the temperatureof this, slurry* is considerably lower .thanmthe urea synthesis temperature, the corrosive vcharacter ofthe vwithdrawn slurry is considerably lessthan aurea synthesis melt at synthesis condition s `v"My process vis a marked vimproverrierit -over. Dror. .art .processesfsirie in those .processes thefsynthesis melt* isf-withdrawn from the' reactor at the synthesis temperature, the

reactor at a temperature lowerA than the u ,thesis temperatura( itr isL necessary @to `cool fthe .entire :melt priorito. ,rernclzvall -from the reactor.

`v`Ineadditionjto employing a nov'el method for producing ,urea, .Ii also f employ a :novel method for removing. the .urea from the synthesis reactor which advantageously i utilizes lan arrangement f .valves similar to that shown 191th ,panying drawing, 'I'hepressureof Vthe,urea lon"- ,taining'slurry in the lowerportion of the reactor is ,still at the 'pressure .employed for the d x thesis freactin` which is usually at lleastashigh as the".autogenousV pressure ldevsloped by themelt. To remove the synthesis, melt lf'ro'm ,thereactorl for separation Oithe'ure'a presentsja; perplexing problem, and prior art processes Q have, employed However, in my process I remove the urea-containing slurryfrom the reactor through a valve which is constructed to withstand the high synthesis reaction pressure into a depressurizing zone which is at substantially the same pressure as the synthesis reaction pressure. After the ureacontaining slurry has been removed from the reactor, the valve isrclosed, and the pressure on the slurry is reduced to about atmospheric pressure. The urea is then readily removed from the system without a large diierential pressure across an open valve.

Referring to the gure, .reactor- I is usually a leador silver-lined, vertically elongated'reaction chamber or autoclave operated at a pressure of 1700 pounds per square inch for the synthesis of urea. Liquid ammonia and liquid carbon dioxide are introduced to the system via Vline 2, and mixtures of the two reactants are intro- `ducedto reactor I multipointwisevia lines 3, 4,

5 and 6. The reactants may enter the reactor at only a single point, but multipoint injection is preferred since the injection of reactants at various points in the reactorvserves as a meansfor obtaining better temperature` control of the reaction mixture and easier heat removal from the reaction mixture. The ammonia and carbon dioxide are usually employed in the reactor in a Ymolar ratio of 2:1 to 4:1, but higher molar ratios, for example :1 and higher, may be employed. The overall'net reaction taking place in reactor I is exothermic, and the reaction temperature can be controlled to some extent by using suitable temperatures of the influent reactants. However, in addition to controlling the temperature in this manner, exothermic heat of reaction can be dissipated by circulating a suitable liquid coolant, such as normally liquid hydrocarbons or Water, through cooling tube or conduit 1. The upper portion of the reactor is thus maintained at a temperature within the range of 300 to 400 F. The lower portion of reactor I isalso provided with cooling tube or conduit 8 through which a liquid coolant similar to that employed in cooling tube 'I is circulated. Cooling tube 8 cools the lower portion of reactor I to a temperature sufficiently low to permit the formation of urea crystals. The preferred temperature is within` the range of 100 to 120 F. The urea crystals form a slurry in the lower portion of reactor I, and they are Withdrawn as described hereinbelow.

Instead of employing cooling tubes for maintaining the desired temperature levels in reactor I, the reactor may be jacketed, and suitable liquid cool-ants are then employed in the jackets surrounding the reactor to remove excess exothermic heat of reaction.

I The proportionate volume of the total volume of the contents of reactor I that is maintained at a temperature suiiciently low to effect the crystallization of urea is dependent primarily upon the design of the reactor. In the figure cooling tube 8 would maintain -about 25` volume .per cent of contents of reactor I at a temperature sucient to cause urea crystallization and about 75 volume per cent .of the contents are maintained at a higher temperature, but this proportion isrsubject to variation. Ordinarily, at least 50 volume per cent of the reaction mixture is maintained at a temperature above the urea crystallization temperature, and no more than 50 volume per cent ofthe reaction mixture is maintained at a temperature sufiicient to cause crystallization of urea. Preferably, from 50 to 90 volume per cent of the reaction mixture is at the higher temperature and from 10 to 50 volume per cent is at the lower temperature.

In order to prevent the accumulation of gases. particularly inert gases, in reactor I, gases therein are Withdrawn via line 9. The gas thus removed from the system contains, in addition to gaseous ammonia and carbon dioxide, inert gases that entered the system with the reactants via line 2. Typical examples of the inert gases are nitrogen and hydrogen and small amounts of the inert gases found in air. The gas passing via line 9 may be discarded but it may be economically desirable to separate the ammonia and carbon dioxide for recycling to reactor I by means not shown. Also, a small portion of this gas may be employed, :as described hereinbelow, in depressuring zone I0.

As a preliminary to the removal of the ureacontaining slurry from reactor I, the pressure in depressurizing zone I0 is raised to not more than 10 to 30 pounds per square inch lower than the pressure in reactor I. This increase in pressure is effected by introducing a gas under pressure via line II and valve I2 While keeping valves I3, I4 and I5 in a closed position. The gas that is employed to eliect this pressure increase may be a small amount, say 0.5 to 5.0 volume per cent, of the gas that is removed from reactor I via line 9. Also, this gas may be one of the urea synthesis reactants, such as carbon dioxide, or it may be an inert gas, such as nitrogen or hydrogen, that has been compressed to the desired pressure. After the desired pressure has been attained in depressurizing zone I0, valve I2 is closed and valve I3 is opened permitting the passage of ureacontaining slurry therethrough. Subsequently, valve I3 is closed and the pressure in zone I0 is reduced to about atmospheric pressure by opening valve I5 in line I6 which is above the liquid level in the depressurizing zone. The gas that is emitted from zone I0 via line I6 contains gas that was employed to build up the pressure in zone I0 along with gaseous ammonia and carbon dioxide, and the gas passing via line I 6 may be sent to a suitable recovery step (not shown) to recover the ammonia and carbon dioxide for use in reactor I. After zone I0 has been depressurized, valve I4 iS opened and the urea-containing slurry passes via line I1 to surge tank I8. From tank I8 a gas is vented via line I9, and, since this gas contains ammonia and Carbon dioxide resulting from the decomposition of ammonium carbamate this gas may also be passed to suitable means (not shown) for recovery of the ammonia and carbon dioxide for use in reactor I.

From surge tank I8 urea-containing slurry passes via line 20 to separating means 2'I, which may be a centrifuge, a filter press or other suitable means for separating the urea crystals from the mother liquor. Urea is withdrawn from separator 2l via line 22, and mother liquor containing water and ammonium carbamate are returned to reactor I via line 23. In order to prevent the excessive accumulation of water in reactor I, a portion of the mother liquor passing via line 23 is withdrawn via line 24 and passed to water removal zone 25, Which may be an evaporator or other suitable means for removing water. Mother liquor having a. decreased Water content passes from zone 25 via line 26, and it is returned to reactor I via line 23.

From the above disclosure modifications of my process within the scope of 'my invention will be the interaction of ammonia and carbon dioxide at an elevated temperature `and pressure, the improvement which comprises maintaining the upper portion of the reaction mixture at an elevated temperature suitable for the synthesis reaction, maintaining the remaining lower portion of the reaction mixture at a temperature below the crystallization point of urea, and during the reaction withdrawing from said lower portion of the reaction mixture solid urea, ammonium carbamate and water, separating the solid urea from the mother liquor, separating said liquor into two portions one of which is treated to remove water and directly recycling the other portion to the reaction without decomposition of the ammonium carbamate therein contained.

2. In a process wherein urea is being formed by the interaction of ammonia and carbon dioxide at an elevated temperature and pressure, the improvement which comprises maintaining at least 5'0 volume per cent of the reaction mixture at a temperature within the range of 300 to 400 F., maintaining no more than 50 volume per cent of the reaction mixture at a temperature below 140 F., and during the reaction withdrawing from the cooler portion of the reaction mixture solid urea ammonium carbamate and water, separating the solid urea from the mother liquor, separating said liquor into two portions, one of which is treated to remove water and directly recycling the other portion to the reaction without decomposition of the ammonium carbamate therein contained.

3. An improvement according to claim 2 wherein from 50 to 90 volume per cent of the reaction mixture is maintained at a temperature within the range of 340 to 400 F.

4. An improvement according to claim 2 wherein from 10 to 50 volume per cent of the reaction mixture is maintained at a temperature within the range of 100 to 120 F.

5. In a process wherein urea is being formed by the interaction of ammonia and carbon dioxide at an elevated temperature and pressure, the improvement which comprises maintaining the upper portion, at least 50 volume per cent, of the reaction mixture at a temperature within the range of 340 to 400 F., maintaining the lower portion, no more than 50 volume per cent, of the reaction mixture at a temperature below F., during the reaction withdrawing a urea-containing slurr'y from the cooler portion of the reaction mixture into a depressurizing zone at a pressure not substantially lower than the pressure of the reaction mixture, reducing the pressure on the thus-withdrawn slurry, recovering solid urea from said slurry to obtain a motor liquor containing ammonium carbamate and water, separating said liquor into two portions, one of which is treated to remove water and directly recycling the other portion to the reaction without decomposition of the ammonium carbamate therein contained.

6. An improvement according to claim 5 wherein the pressure in the depressurizing zone into which the urea-containing slurry is withdrawn is not more than 30 pounds per square inch lower than the pressure 0f the reaction mixture.

7. An improvement according to claim 5 wherein the interaction of ammonia and carbon dioxide is eiected at the autogenous pressure of the reaction mixture and wherein the pressure in the depressurizing zone into which the urea-containing slurry is withdrawn is equal to said autogenous pressure prior to reduction of the pressure on said slurry.

8. An improvement according to claim 5` wherein the pressure on the urea-containing slurry is reduced to atmospheric pressure in the depressurizing zone.

9. An improvement according to claim 5 wherein liquid from the solid urea recovery step is recirculated to the reaction mixture.

DONALD H. WHITE.

REFERENCES CITED The following references are of record in th iile of this patent:

UNITED ySTATES PATENTS Number Name Date 1,670,341 Casale May :22, 1928 1,782,723 Hetherington et al. Nov. 25, 1930 2,038,564 Hetherington et al. Apr. 28, 1936 FOREIGN PATENTS Number Country Date 397,681 Germany July 1, 1924 

1. IN A PROCESS WHEREIN UREA IS BEING FORMED BY THE INTERACTION OF AMMONIA AND CARBON DIOXIDE AT AN ELEVATED TEMPERATURE AND PRESSURE, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE UPPER PORTION OF THE REACTION MIXTURE AT AN ELEVATED TEMPERATURE SUITABLE FOR THE SYNTHESIS REACTION, MAINTAINING THE REMAINING LOWER PORTION OF THE REACTION MIXTURE AT A TEMPERATURE BELOW THE CRYSTALLIZATION POINT OF UREA, AND DURING THE REACTION WITHDRAWING FROM SAID LOWER PORTION OF THE REACTION MIXTURE SOLID UREA, AMMONIUM CARBAMATE AND WATER, SEPARATING THE SOLID UREA FROM THE MOTHER LIQUOR, SEPARATING SAID LIQUOR INTO TWO PORTIONS ONE OF WHICH IS TREATED TO REMOVE WATER AND DIRECTLY RECYCLING THE OTHER PORTION TO THE REACTION WITHOUT DECOMPOSITION OF THE AMMONIUM CARBAMATE THEREIN CONTAINED. 